Microgravitational effects on the neurovestibular system and countermeasures to facilitate safe and effective adaptation to changes in gravity during future Mars explorations

1.1 Introduction With the Artemis program National Aeronautics and Space Administration (NASA) plans to land humans on the moon to prepare for the next giant leap, Mars. In order to achieve this ambitious goal, lots of medical and physiological obstacles have to be overcome to ensure a safe voyage. A common but manageable issue with space travel has been neurovestibular dysfunction and altered sensorimotor functions, resulting in symptoms of space motion sickness (SMS) and spatial disorientation. With increasing spaceflight duration and adaptation to various gravities, this hurdle becomes more prominent and will require effective countermeasures. 1.2 Purpose of the study The purpose of this study is to get a closer look at how microgravity affects the neurovestibular system, and to consider which countermeasures that can assist in a safe and effective adaptations to gravitational changes. 1.3 Material and method By executing a systematical literature search through the databases PubMed and MEDLINE, articles were found and included in the thesis based on carefully selected criteria. The main search in PubMed resulted in 1780 articles, which first went through a filtration to exclude articles in other languages than English, published before 2011 and with no available abstract or full text. 132 articles’ title and abstract were then reviewed for relevance based on the selection criteria. 94 articles were excluded, and the remaining 36 articles were read in full text. Of these, 19 articles were relevant and contributed to the purpose of the study, and therefore included in the thesis. To cover articles not found in PubMed, another search was carried out in MEDLINE, providing an additional 4 articles. 1.4 Results Studies has shown that space motion sickness and spatial disorientation mainly affect astronauts during and right after gravitational transitioning. When being exposed to microgravity for longer durations, adaptation mechanisms are activated and leads to utricular deconditioning with synaptic plasticity and decreased ocular counter-roll reflex. Susceptibility to space motion sickness seems to be higher in females, astronauts with a visual-depended orientation preference and increased otolith mass asymmetry. Pharmaceuticals, especially scopolamine and promethazine, are widely used to prevent or to ease symptoms of SMS. The effect of these drugs is highly variating. Scopalamine has been found to be quite efficient in lower the symptoms, but has shown side effects of drowsiness, which could be counteracted by combining scopolamine with dextroamphetamine (ScopeDex). Artificial gravity (AG) is suggested as a possible and effective countermeasure against vestibular deconditioning and intermittent exposures has been proven to be more tolerable. Virtual reality has been tested and showed promising results in modifying orientation preference to be less depended on visual cues. Galvanic vestibular stimulation is an analog creating similar altered vestibular inputs as microgravity and can be used in preflight adaptation training. 1.5 Conclusion Microgravity can induce space motion sickness and spatial disorientation in astronauts, especially during gravitational transitioning. Long-duration exposure also leads to utricular deconditioning with synaptic plasticity and decreased ocular counter-roll reflex, which contributes to reentry symptoms. Susceptibility to SMS variates and can be mapped with Motion Sickness Susceptibility Questionnaire, as well as vestibular and visual tests. Females seems to be more susceptible, due to their orientation preference being more visual-dependent. The sensory conflict, which leads to SMS, has been shown to increase in astronauts with marked otolith mass asymmetry. Countermeasures include the use of pharmaceuticals, with individual and limited effect, and preflight training to aid adaptation through galvanic vestibular stimulations, virtual reality and artificial gravity. There is a call for future research to develop countermeasures that are both effective and affordable